U.S. patent application number 14/848536 was filed with the patent office on 2016-03-10 for superconducting magnet device including a cryogenic cooling bath and cooling pipes.
This patent application is currently assigned to SIEMENS AG. The applicant listed for this patent is Siemens Aktiengesellschaft. Invention is credited to Neil Charles Tigwell.
Application Number | 20160071638 14/848536 |
Document ID | / |
Family ID | 51796381 |
Filed Date | 2016-03-10 |
United States Patent
Application |
20160071638 |
Kind Code |
A1 |
Tigwell; Neil Charles |
March 10, 2016 |
SUPERCONDUCTING MAGNET DEVICE INCLUDING A CRYOGENIC COOLING BATH
AND COOLING PIPES
Abstract
In a method for cooling a superconducting magnet device suitable
for magnetic resonance imaging, and a cooling system, a small
quantity of cryogen is used by cooling the magnet coils of the
magnet device by a cooling pipe assembly, the cooling pipe assembly
having one or more cooling pipes through which a cryogen flows. The
one or more cooling pipes are in close thermal contact with the
magnet coils. Liquid cryogen is filled into a cryogen vessel to
provide a cryogenic temperature for at least parts of the magnet
device for superconducting operation. The magnet coils are cooled
during energizing of the magnet device by this cooling pipe
assembly.
Inventors: |
Tigwell; Neil Charles;
(Witney, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Muenchen |
|
DE |
|
|
Assignee: |
SIEMENS AG
Muenchen
DE
|
Family ID: |
51796381 |
Appl. No.: |
14/848536 |
Filed: |
September 9, 2015 |
Current U.S.
Class: |
505/163 ;
335/216 |
Current CPC
Class: |
H01F 6/02 20130101; H01F
6/04 20130101; F17C 3/085 20130101 |
International
Class: |
H01F 6/04 20060101
H01F006/04; F17C 3/08 20060101 F17C003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2014 |
GB |
1415880.2 |
Claims
1. A superconducting magnet device comprising: magnet coils; a
cryogen vessel housing the magnet coils; a cooling pipe assembly
comprising at least one cooling pipe in thermal contact with the
magnet coils; and the cooling pipe assembly being arranged within
the cryogen vessel and comprising an inlet pipe and an outlet pipe,
said inlet pipe and outlet pipe terminating outside the cryogen
vessel.
2. The superconducting magnet device as claimed in claim 1, wherein
the at least one cooling pipe of the cooling pipe assembly forms
multiple cooling loops.
3. The superconducting magnet device as claimed in claim 1, wherein
the at least one cooling pipe of the cooling pipe assembly forms a
closed loop.
4. The superconducting magnet device as claimed in claim 1,
comprising a high pressure cooler and outside the cryogen vessel,
connected to said cooling pipe assembly by said inlet pipe and
outlet pipe.
5. The superconducting magnet device as claimed in claim 1,
comprising a supply vessel and outside the cryogen vessel,
connected to said cooling pipe assembly by said inlet pipe and
outlet pipe.
6. The superconducting magnet device as claimed in claim 1, wherein
at least one part of said cooling pipe assembly is connected as a
current lead to supply electrical current to the magnet coils.
7. A method for cooling a superconducting magnet device comprising
magnet coils in a cryogen vessel, said method comprising: cooling
the magnet coils before energizing the magnet coils with a cooling
pipe assembly, by introduction of a cryogen into the cooling pipe
assembly through an inlet pipe and out through an outlet pipe that
both terminate outside of said cryogen vessel; partially filling
the cryogen vessel with a liquid cryogen to provide a cryogenic
temperature to at least parts of said magnet coils for
superconducting operation; and cooling the magnet coils during
energizing of the magnet coils by further introduction of a cryogen
into said cooling pipe assembly through said inlet pipe (14) and
out through said outlet pipe.
8. The method as claimed in claim 7, comprising cooling the magnet
coils before said energizing thereof using a cooler to provide the
cooling pipe assembly with the cryogen.
9. The method as claimed in claim 7, comprising cooling the magnet
coils during said energizing thereof the magnet coils using a
cooler to provide the cooling pipe assembly with the cryogen.
10. A method as claimed in claim 7, comprising cooling the magnet
coils before said energizing thereof with cryogen provided to said
cooling pipe assembly from a supply vessel.
11. A method as claimed in claim 7, comprising cooling the magnet
coils during said energizing thereof with cryogen provided to said
cooling pipe assembly from a supply vessel.
12. The method as claimed in claim 7, comprising cooling the magnet
coils during energizing, using at least one part of the cooling
pipe assembly as a current lead to supply electrical current to the
magnet device.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention concerns a method for cooling a
superconducting magnet device suitable for magnetic resonance
imaging. Furthermore, the present invention concerns a cooling
system for a superconducting magnet device suitable for magnetic
resonance imaging.
[0003] 2. Description of the Prior Art
[0004] Superconducting magnet devices are used for medical
diagnosis, for example in magnetic resonance imaging (MRI) systems.
A requirement of an MRI magnet is that it produces a stable,
homogeneous, magnetic field. In order to achieve the required
stability, it is common to use a superconducting magnet device
which operates at very low temperature. The temperature is
typically maintained by cooling the superconductor by means of a
low temperature cryogenic fluid, also known as a cryogen, such as
liquid helium.
[0005] The superconducting magnet device typically comprises a set
of superconductor coils for producing a magnetic field, the coils
being immersed in a cryogenic fluid to keep the coils at a
superconducting temperature, the superconductor coils and the
cryogen being contained within a cryogen vessel. For this purpose,
the vessel contains a large amount of helium in order to provide a
helium bath for large parts of the magnet coils. In other
arrangements, the superconducting magnet is configured as minimum
helium magnet, which preferably does not require cooling in a
helium bath. Instead the magnet is cooled using much smaller
quantities of cryogen.
[0006] Superconducting magnets are susceptible to quench events, in
which, for a number of reasons, part of the superconducting magnet
ceases to be superconducting. The resulting resistance in part of
the magnet causes heat due to the current flowing through it. This
rapidly causes further parts of the superconducting magnet to cease
superconducting. The result is that all of the energy which was
stored in the magnetic field of the magnet is suddenly released as
heat. In a superconducting magnet cooled by a liquid cryogen, this
typically results in rapid boil-off of a large volume of the
cryogen, with gaseous and liquid cryogen being expelled from the
cryostat at high speed.
[0007] During the process of supplying electrical current to the
coils of the superconducting magnet ("ramping") there is a
potential risk of quenching events, in particular if a minimum
helium magnet is employed. In case of minimum helium magnets it has
been suggested to use displacers in the cryogen vessel to provide a
greater wetted area for a given volume of helium.
[0008] For all types of magnets it is desirable to be able to
transport the magnet device to the operational site in a dry,
possible warm state, e.g. by sea shipment. However, once arrived at
site it is necessary to cool the magnet down again. Typically, for
this purpose, the vessel is filled in a first step with nitrogen.
After a sufficiently low temperature has been reached, the nitrogen
is blown out in a second step and the vessel is filled with liquid
helium to provide cryogenic temperatures to the magnet coils in a
third step. After the required temperature for ramping the magnet
has been reached, all cooling is stopped, allowing the magnet to
depressurise prior to ramping.
SUMMARY OF THE INVENTION
[0009] As helium becomes more expensive and less readily available,
it is desirable to reduce the amount of helium needed to cool down
the superconducting magnet device after shipment and to operate a
superconducting magnet device. It is therefore an object of the
present invention to provide a simple technique for cooling a
superconducting magnet device using a comparatively small quantity
of cryogen.
[0010] This object is achieved according to the invention by a
method for cooling a superconducting magnet device suitable for
magnetic resonance imaging, that includes the steps of cooling down
the magnet coils of the magnet device with a cooling pipe assembly,
the cooling pipe assembly having one or more cooling pipes through
which a cryogen flows, these one or more cooling pipes being in
close thermal contact with the magnet coils, filling a liquid
cryogen into a cryogen vessel to create a cryogenic temperature in
at least parts of the magnet device for superconducting operation,
and cooling the magnet coils during energizing of the magnet device
by the cooling pipe assembly.
[0011] The object of the present invention is also achieved by a
cooling system for a superconducting magnet device suitable for
magnetic resonance imaging, having a cryogen vessel to contain a
liquid cryogen reservoir to create a cryogenic temperature in at
least parts of the magnet device for superconducting operation, and
a cooling pipe assembly, the cooling pipe assembly having one or
more cooling pipes that are in close thermal contact with the
magnet coils of the magnet device, the cooling pipe assembly being
designed to contain a cryogen to cool the magnet coils of the
magnet device and to cool the magnet coils during energizing of the
magnet device.
[0012] A basis of the invention is to provide a second cooling
arrangement, that is operable completely independent of the liquid
cryogen reservoir contained within the cryogen vessel. This second
cooling arrangement in form of a cooling pipe assembly being in
close thermal contact with the magnet device, and can be used to
cool the magnet device after shipment of the magnet device in a dry
cryogen vessel. More importantly, the second cooling arrangement
can be used during ramping to cool the magnet in a cryogen filled
vessel.
[0013] These and other aspects of the invention will be further
elaborated on the basis of the following embodiments.
[0014] The cooling pipe assembly can be operated either using a
cooling machine, such as a mechanically operated cooling machine,
or using a supply vessel, for example a Dewar vessel. Despite that
a cryogen vessel is typically of low pressure construction, with
the present invention a high pressure mechanical cooling system can
be used for cooling the magnet device, since the cooling pipes are
preferably made in a way to withstand high pressure. Preferably,
relatively narrow bore metal pipes are employed, which can stand
very high pressures with no problems.
[0015] It is of further advantage that the cooling pipe assembly
can be operated using a gaseous cryogen or a liquid cryogen, as
required for an ideal cooling functionality. Furthermore, the mode
of operation and the cooling medium can be changed without
difficulty during a cooling process. For example, a high pressure
cooler may be employed for cooling down the magnet by the cooling
pipe assembly, thereby using nitrogen gas.
[0016] Subsequently, a Dewar vessel may be employed for further
cooling down the magnet by the cooling pipe assembly, thereby using
liquid helium. After a sufficiently low temperature has been
reached, the magnet is energized. At the same time, the cooling
pipe assembly may be employed to cool the magnet to prevent a
ramping quench event.
[0017] Advantageously, the cooling process by operation of the
cooling pipe assembly is preferably not interrupted or stopped
during the step of filling cryogen into the cryogen vessel. It is
of particular advantage, that the ramping process can take place
immediately following the filling step. At this particular time,
all relevant structural parts of the system are at their coldest,
which eliminates any risk of cryogen boil-off Furthermore, there is
no need to stop the cooling process, e.g. in order to depressurize
the magnet prior to ramping. Because cooling by means of the
cooling pipe assembly can be carried out during the complete
ramping cycle, any heat generated by the magnet is extracted. The
risk of quenching is thereby considerably reduced.
[0018] The present invention is particularly useful for minimum
helium magnets, since the cooling effect of a minimum cryogen
reservoir is increased by operation of the cooling pipe assembly,
which provides a direct cooling to parts of the magnet device to
which they are in close thermal contact.
[0019] In prior art magnet systems, cooling of the current lead, in
particular cooling of the positive electrical connector, is often
problematic. By means of the invention, a very efficient cooling of
a current lead can be achieved in a simple manner by using a part
of the cooling pipe assembly itself as current lead to supply
electrical current to the magnet. Therefore, no additional
electrical connections, e.g. conductors leading through the access
neck, have to be provided, which would result in additional heat
load to the magnet. The circulating cryogen will maintain the
current lead at low temperature without any effect on magnet
pressure or thermal environment, as it is the case in prior
art.
[0020] As the cooling pipe assembly is housed within the cryogen
vessel, it does not need to be perfectly leak tight, which allows a
simple, low cost construction. With the present invention a simple
technique is provided for cooling a superconducting magnet device
using a comparatively small quantity of cryogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 schematically illustrates a cryostat.
[0022] FIG. 2 shows a detail of the electrical connection to the
magnet device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] FIG. 1 shows a cryostat 1 such as may be employed for
holding a magnet device for an MRI (magnetic resonance imaging)
system. In the illustrated embodiment, a minimum helium magnet is
employed, i.e. the magnet does not require cooling in a helium
bath, instead parts of the magnet are cooled directly using small
quantities of helium.
[0024] A cryogen vessel 2 holds a liquid cryogen 3, e. g. liquid
helium. During operation, the helium reservoir is at a maximum
cryogen level 4. The cryogen vessel 2 is contained in a vacuum
jacket 5. 5 One or more heat shields (not shown) may be provided in
the vacuum space between the cryogen vessel 2 and the vacuum jacket
5. A turret 6 with an access neck 7 is provided near the bottom of
the cryostat 1, allowing access to the cryogen vessel 2 from the
exterior. This is used to fill the cryogen vessel 2 and to provide
access for connections to superconductive magnet coils 8 housed
within the cryogen vessel 2. Besides the magnet coils 8, the magnet
device has a termination board 9, with switches and other parts for
operating the magnet. From the termination board 9 conductors (not
shown) lead to the magnet coils 8.
[0025] Since a minimum helium magnet is used, the termination board
9 is immersed in the helium reservoir during operation of the
magnet, thereby being cooled directly. For cooling the magnet coils
8 during operation a cold finger 11 is employed, which projects
into the helium reservoir.
[0026] The cooling system of the magnet device has a cooling pipe
assembly 12 with a cooling pipe 13 arranged within the cryogen
vessel 2. The cooling pipe 13 is a relatively narrow bore pipe made
of metal, preferably made of copper or stainless steel. The cooling
pipe 13 is designed as a closed loop and comprises an inlet 14 and
an outlet 15. The cooling pipe 13 forms a number of cooling loops
(windings) around the magnet coils 8, thereby being in close
thermal contact to the magnet coils 8 in a way that allows to
withdraw sufficient heat from the magnet coils 8. Preferably, the
cooling pipe 13 is at least partially in mechanical contact with
the outer surface of the magnet coils 8.
[0027] In FIG. 1, for clarity, only a single winding is depicted.
Typically, a large number of windings are provided. Generally, the
cooling loops can be realized either as a continuous pipe
arrangement or as a split pipe, e.g. with an optimized flow
design.
[0028] The cooling system further has a high pressure mechanical
cooling machine and a Dewar vessel (both not shown), each being
connectable via the turret 6 to inlet 14 and outlet 15 of the
cooling pipe 13. The connections can be achieved via detachable
vacuum insulated tubes connecting at the thermal interface 16,
which is arranged to intercept radiation and conduction heat loads
to the cryogen vessel 2. In the illustrated embodiment inlet 14 and
outlet 15 terminate at the turret outer jacket 17.
[0029] After transportation of such a cryostat 1 in a dry and warm
state, a cooling process is started by connecting the high pressure
cooler to bring the magnet temperature down to about 50 K. This
pre-cooling procedure is continued using liquid helium from a Dewar
vessel. Toward the end of the cooling process, the cryogen vessel 2
is filled from the Dewar vessel, e.g. be opening an additional
valve (not shown), via a conventional siphon tube 18 which projects
into the cryogen vessel 2, to provide cryogenic temperatures to at
least parts of the magnet for superconducting operation. After a
sufficiently low temperature for ramping the magnet has been
reached, normally all cooling would be stopped allowing the magnet
to depressurise prior to ramping. With the invention, however, the
siphon feed can be shut off allowing the pressure in the magnet to
reduce while maintaining the cooling of the magnet directly via the
cooling pipe 13. During ramping the magnet coils 8 are further
cooled by the cooling pipe 13, thereby avoiding the risk of a
ramping quench.
[0030] For providing electrical current to the magnet, a magnet
power supply 19 is used. A negative electrical connection 21 is
provided to the magnet through the body of the cryostat 1. Instead
of providing a positive electrical connection by an additional
conductor, the outlet 15 of the cooling pipe 13 itself is used as
the positive current lead to supply electrical current to the
termination board 9. For this purpose, the outlet terminal 22 is
connected to the positive connection 23 of the power supply 19. As
it is illustrated in FIG. 2, the current lead section 24 of the
outlet 15 is connected to a cable 25 which establishes the electric
connection between the outlet 15 and the termination board 9.
[0031] Following the connecting point 26 of the cable 25, towards
the interior of the cryogen vessel 2, the current lead section 24
of the outlet 15 is electrically isolated by means of a suitable
isolator 27, which for example is made from ceramic material.
Additional pressure may be required at the Dewar vessel to overcome
any boiling effect of helium in the current lead section 24. This
would be helped by maintaining a very low pressure outlet for the
exhausted helium gas. As the quantity of gas would be small it is
possible to vent via a low pressure check valve direct to
atmosphere.
[0032] It will be evident to those skilled in the art that the
invention is not limited to the details of the foregoing
illustrative embodiments, and that the present invention may be
embodied in other specific forms without departing from the scope
of the invention. For example, the cooling pipe assembly may be
used for a conventional flooded magnet device, thereby allowing
additional cooling as well. Instead of helium, other cryogens may
be used, for example nitrogen, hydrogen or a combination
thereof.
* * * * *